Include unspecified additional properties in structure layout.

[timothee-haudebourg]

For now TreeLDR structure layouts are composed of a list of expected fields. Additional unspecified fields appearing in an instance of such layout will be rejected (and an error raised). However it is sometime useful to capture additional unspecified fields in the layout, for instance like so:

layout Resource for rdf:Resource {
  rdfs:label: String,
  * as anyOtherProperty: rdf:Resource
}

This special * field will catch any additional property in a map in the generated code, here in Rust:

struct Resource {
  label: Option<String>,
  any_other_property: HashMap<Id, Resource>
}